Possession is a novel about letters: their discovery, their possession, the stories they tell. "They wrote a lot, in those days," says Toby, a lawyer dealing with "the most ferocious wrangle about a correspondence between dead poets that someone's just discovered".

Roland and Maud, the academic sleuths who find the letters of two Victorian poets and lovers, Randolph Ash and Christabel LaMotte, begin with the dull motives of scholars. But the letters they find re-animate the past, and foster their own romance. The key moments of the novel are the discoveries of letters (making Roland and Maud, incidentally, the luckiest academics alive). There they are in a doll's cot, in a shoe box, finally in a grave. Thus the past floods into the present.

Byatt rediscovers a resource that novels now use rarely. The novel can seem literally to give us these letters, true relics laid out on the page. And, as in earlier English novels, they show that entering into a correspondence is dangerous. Letters can be read by the wrong person ("purloined", in Possession, by Christabel's jealous lady "companion"). They are dangerous to the writers themselves: the poets are drawn into love through letters.

In one, Christabel admits that letters become "an Addiction". "That space of freedom", Ash calls their correspondence. While "shy with each other" in person, "in a papery way we knew each other so much better". They are most charged in a world where ideas of propriety are strongest.

Naturally, then, Jane Austen uses them. In Pride and Prejudice, Mr Darcy can only share his confidences with Elizabeth Bennet by pressing a letter into her hand. In Persuasion, Captain Wentworth has to declare his love to Anne Elliot by letter, even though she is sitting in the same room as him.

In novels, letters say things that cannot be spoken. In Possession, Christabel writes to Ellen Ash when her husband is dying to ask her to pass him - what else, but a letter? It is never handed on. If readers have found this deeply academic novel moving, it is because of the letters that do not get read, or sometimes do not get sent. Randolph Ash's letter to heal his breach with LaMotte is found by his wife in the back of his desk and burned.

The novel, since its youth, has needed letters. Novels by Richardson, Smollett, Burney and Laclos were written entirely in letters. When Austen first planned Sense and Sensibility it was to have been in letters. Even after the epistolary convention was largely abandoned, these strange documents, often both formal and revealing, remained important. Think about how Mr Collins's letter of self-introduction comically reveals him in Pride and Prejudice or how Mr Knightley performs a practical criticism of Frank Churchill's letter of self-exculpation in Emma.

Letters are interesting to Byatt for many of the same reasons that they have been to novelists in the past. They reveal the truth through gaps, so that the clandestine courtship of Randolph Ash and Christabel LaMotte happens in between the letters they write to each other. The reader must infer the drama and bring the affair alive from hints.

Above all, letters combine revelation with mystery. A letter seems to give us immediate access to a person, yet must also be distrusted. Sometimes in Possession the distrust is obvious. We read the letters of Ash to his wife Ellen, written while on a trip to north Yorkshire. They are full of local history and amateur geology, and apparent fondness. Yet we know he is there with his lover. The excitement of his prose is not what it must have seemed to Ellen.

From 18th- and 19th-century fiction, Byatt has learnt that a letter is alive when it imagines its intended reader, the single person who will examine it. Her correspondents can even rebel against the very "Victorian" styles that she has fabricated for them. Randolph Ash's letters turn into mockery of his own sombre religious scepticism and slowly his proprieties crumble. "But I write like a sermon preacher", scribbles Christabel, puncturing the Christina Rossetti-like pietism she has just been reproducing. Letters are where the heart cannot be stilled.

· John Mullan is senior lecturer in English at University College London

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